1. Field of the Invention
The present invention relates to a negative electrode active material for nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Owing to the rapid progress in downsizing technology of electronic appliances, recently, various portable electronic appliances are developed and distributed widely. Downsizing is also required in the batteries used as power sources for these portable electronic appliances, and nonaqueous electrolyte secondary batteries having a high energy density are attracting much attention.
A nonaqueous electrolyte secondary battery using metal lithium as negative electrode active material has a very high energy density, but the battery life is short because dendritic crystals called dendrites deposit on the negative electrode at the time of charging. Besides, in the secondary battery, dendrites grow and reach up to the positive electrode to cause internal short-circuit, thereby leading to safety problems. Accordingly, as a negative electrode active material replacing the lithium metal, it has been proposed to use carbon materials, in particular, graphitized material capable of absorbing and desorbing lithium. However, the capacity of graphitized material is small as compared with that of lithium metal or lithium alloy, and large current characteristics are low. It has been also attempted to use substances of larger lithium absorption capacity and high density, such as elements which are alloyed with lithium, for example, silicon and tin, or an amorphous chalcogen compound.
Above all, silicon can absorb lithium up to a ratio of 4.4 atoms of lithium per 1 atom of silicon, and the negative electrode capacity per weight is about 10 times that of graphitized material. However, since silicon is large in change of volume due to intercalation and deintercalation of lithium in the charging and discharging cycles, particles of the active material are finely pulverized, and there were problems in cycle life.
Jpn. Pat. Appln. KOKAI Publication No. 2000-215887 discloses a negative electrode material for lithium secondary battery, comprising a silicon particle nucleus and a carbon layer for covering the surface of the silicon particle nucleus.
According to the publication, the average particle size of silicon particle nucleus is 0.1 to 50 μm. However, if the average particle size of particle nucleus is 0.1 μm or more, it is hard to prevent pulverization and cracking of the active material in the charging and discharging cycle.
The publication does not specify the half width of any diffraction peak at the (220) plane of silicon, determined by the powder X-ray diffraction of negative electrode material. However, in the publication, as silicon particle nucleus, reagent first class silicon powder by Wako Pure Chemical Industries, Ltd. is used. It is known that this powder is crystalline silicon powder. From the description about the material and in view of FIG. 1 of the publication, it can be inferred that the diffraction peak at the (220) plane of silicon has a half width of less than 0.1°.
FIG. 1 of the publication shows the XRD profile of the negative electrode material used in Example 2. FIG. 1 does not represent the entire image of the peak at the (220) plane of silicon. An accurate half width cannot be determined from FIG. 1. Assuming that the maximum peak intensity shown on the axis of ordinates in FIG. 1 is the peak intensity of (220) plane, the present inventors measured the half width. The result was a value smaller than 0.1°.
In the negative electrode active material mentioned in the publication, it is difficult to realize a battery of higher capacity and higher cycle characteristic.